Whenever I visit a city I find traveling by bicycle very convenient. It's very flexible and you get to see the city in a very friendly way.

I was wondering if there is a website or other resource that lists cities in Europe where public bikes are accessible also to tourists.

Note: I am not referring to companies renting bikes. I am really referring to public systems like exist in Paris or Copenhagen where everyone can use bikes.
If the list contains the requirements to use the bicycles that would be a big plus. Eg.: In Paris, if I remember right, you need a Credit Card.